1. Field of the Invention
The present invention relates to image projection systems, and in particular, to a method and apparatus for reducing RF noise generation or required laser output in an image projection system.
2. Introduction
A portable projector or micro-projector can provide an image larger than the hosting projecting device dimension. Such displays have the potential to overcome the display size limitation on mobile devices and can be used to display documents and images, for example. The displays can also help to overcome the input interface size limitation on mobile devices by displaying virtual input devices, such as keyboards. One technology used for micro-projection is laser scanning, in which red, green and blue laser beams are scanned across a surface to form a full color image. To provide the user of the device with optimum mobility and ease of use, these devices must be small in size, lightweight, and battery powered. However, the requirements necessary to achieve portability result in severe constraints on space, weight, and power dissipation, and increase the amount of heat energy generated by the components of the portable device.
In order to accommodate the brightness and image quality requirements, the conventional solutions for minimizing power consumption in laser-scanned projection require an increase in peak laser output power and peak laser drive current. This increase in laser drive current increases the generation of radio frequency (RF) noise. The effects of this additional RF noise could be particularly significant if the projector is integrated within a portable phone. The noise could exacerbate the problem of desense, or loss of receiver sensitivity, leading to a significant degradation of radio performance.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for reducing peak laser output power and associated RF noise generation in laser-scanned projection.
The scanning of the projector laser beam is achieved by one or two mirrors that reflect the beam and move in such a way as to create a raster pattern on a screen. The raster is created by a horizontal fast movement and a slower vertical movement. The vertical movement is linear, creating on the screen a spot that moves at constant velocity. The mirror creating the horizontal movement oscillates in a resonance mode which enables a much faster movement of the spot on the screen. The penalty of resonance movement is that the mirror moves as a harmonic oscillator and, consequently, the spot velocity is sinusoidal. This requires a higher light output from the lasers during the time when the mirror moves faster. The higher current required for driving the lasers during this time can generate higher RF noise. Therefore, there is a need to make the speed of the horizontal movement of the mirror essentially constant, to minimize the generation of excessive RF noise.
The patent to Barnea (U.S. Pat. No. 7,042,613) describes a method of linearizing the movement of the mirror consisting in a bouncing mechanism that reverses the movement at the end of its range. This mechanical solution complicates considerably the construction of the mirror.
The patent to Bridgelall (U.S. Pat. No. 5,889,269) describes an optical scanner for reading two-dimensional indicia such as bar code symbols and the like and includes a scan element arranged to create a two-dimensional raster pattern which may be scanned across the indicia to be read. In order to improve linearity of response in the vertical direction, perpendicular to the direction of the scan lines, the scan element is driven in that direction by a modified triangular waveform in which the amplitude and/or the phase of at least one of the harmonics has been altered. However, this prior art only applies linearization orthogonal to the fast scanning direction and not parallel to the fast scanning direction. And this prior art is only applied to improve the linearity of response within a two-dimensional raster patter across indicia to be read, and not to minimize RF noise generation associated with image projection near high RF sensitivity communications equipment.
It is the object of this invention to provide a means of reducing the laser peak power, peak laser drive current, and generated RF noise by improved linearity of the mirror movement.